This invention relates to the stabilization of liquid sulfur trioxide and particularly to the stabilization of liquid sulfur trioxide containing small quantities of H.sub.2 SO.sub.4.
At room temperature, liquid sulfur trioxide polymerizes to various degrees. The three generally recognized SO.sub.3 modifications melt at approximately 17.degree.C., 32.5 C. and 62.degree.C., respectively. On standing at temperature below about 27.degree.C., in the presence of even small amounts of moisture, the form melting at about 17.degree.C. rapidly polymerizes to the higher melting forms. The mass soon becomes solid and must be melted at temperatures up to 100.degree.C., usually under dangerously high pressures.,
In many industrial processes, it is highly desirable to use SO.sub.3 in its strongest form. However, the foregoing characteristics of liquid sulfur trioxide create marked disadvantages relative to the storage, shipment and use of such material. While, for example, liquid sulfur trioxide might be shipped in drums equipped with heating coils, there is involved the problem of supplying a relatively expensive form of container. Moreover, since heat transfer through a solid material is poor, the solidified sulfur trioxide in the container can be melted by the user only with considerable difficulty. In addition to this disadvantage, after relatively long periods of standing, a large proportion of the liquid sulfur trioxide will have polymerized to the high melting forms in which case complete melting can only be effected by heating at high temperatures under considerable pressure.
Various materials have been added to liquid sulfur trioxide to prevent the formation of solid polymers. Although boron trioxide has been successfully used commercially as a stabilizer, it leaves a solid residue after vaporization of the stabilized liquid product. Moreover, the product requires subsequent pressure treatment for effective stabilization. Use of trimethoxy boroxine or methyl borate gives a product leaving a non-flowable gummy or viscous residue after vaporization. Such residues adhere to the walls of the vaporizer, necessitating periodic cleaning of the vaporizer, e.g., by water washing, and subsequent careful drying to avoid undesirable introduction of water into the liquid sulfur trioxide. Use of polymeric methyl or methoxy substituted siloxanes is also effective and yields a liquid residue on evaporation. Unfortunately, however, such siloxanes also have relatively low tolerance for H.sub.2 SO.sub.4, and relatively large amounts of such siloxanes are required, in the order of about 0.5 percent by weight, to stabilize sulfur trioxide containing H.sub.2 SO.sub.4 in amount of about 0.1 percent by weight. This is undesirable from a commercial point of view since these siloxanes are rather expensive. Use of dimethyl sulfate, although giving a product leaving a flowable liquid residue after vaporization, fails to stabilize liquid sulfur trioxide containing as little as about 0.01% H.sub.2 SO.sub.4 (equivalent to about 0.002% H.sub.2 O) and often fails to stabilize completely anhydrous liquid sulfur trioxide. Use of a combination of boron compound and dimethyl sulfate is quite effective in stabilizing liquid sulfur trioxide. However, dimethyl sulfate is highly toxic and a suspected carcinogen so that its use is desirably avoided.
Hence, there is a distinct need in the art to develope stabilizers for liquid sulfur trioxide which give a product which leaves a flowable liquid residue after vaporization, does not require subsequent pressure treatment for effective stabilization, has good moisture tolerance, has relatively low toxicity and is of relatively low cost.
An object of this invention is to provide stabilized forms of liquid sulfur trioxide which do not polymerize to any substantial degree and remain liquid at room temperature.
Another object of the invention is to provide stabilizing agents which have a high tolerance for moisture, i.e., which are effective for stabilizing liquid sulfur trioxide containing small quantities of sulfuric acid.
Still another object of the invention is to provide stabilizing agents which result in a product not requiring subsequent pressure treatment to effect stabilization.
A further object of the invention is to provide stabilizing agents which leave a flowable liquid residue after vaporization of the stabilized liquid sulfur trioxide and which have relatively low toxicity.